The 3GPP (3rd Generation Partnership Project) has established an EUTRAN (Evolved UMTS Terrestrial Radio Access Network) (UMTS: Universal Mobile Telecommunications System) network that is referred to as Dual Connectivity, in which packet data are transmitted and received between two eNBs (eNode Bs) and a UE (User Equipment).
FIG. 18 illustrates an example of a configuration of a wireless communication system implementing Dual Connectivity.
The wireless communication system illustrated in FIG. 18 includes a UE 1, an MeNodeB (Master eNode B. Hereinafter, written as MeNB) 2, an SeNodeB (Secondary eNode B. Hereinafter, written as SeNB) 3, an MME (Mobility Management Entity) 4, and an S-GW (Serving Gateway) 5.
The MeNB 2 is a master cell base station.
The SeNB 3 is a secondary cell base station. Note that a cell (SCG: Secondary Cell Group) under control of the SeNB 3 is located within a coverage area of a cell (MCG: Master Cell Group) under control of the MeNB 2.
The UE 1 is a terminal device that receives DL (DownLink) packet data from the MeNB 2 and the SeNB 3. Note that the UE 1 transmits UL (UpLink) packet data to only the MeNB 2, or to the MeNB 2 and the SeNB 3.
The MME 4 is a core network device arranged in a CN (Core Network), and manages C (Control)-plane transmission and movement of the UE 1.
The S-GW 5 is a core network device arranged in the CN, and transmits U (User)-plane packet data.
Note that the MeNB 2 is connected with the SeNB 3 through an X2 Interface, and the MME 4 and the S-GW 5 are connected with the MeNB 2 and the SeNB 3 through an S1 Interface.
FIG. 19 illustrates an example of a C-plane connection configuration in Dual Connectivity.
C-plane connection is made as illustrated in FIG. 19. Connection of the UE 1 in a connection state of Dual Connectivity is only S1-MME between the MeNB 2 and the MME 4. In addition, RRC (Radio Resource Control) Connection of the UE 1 is present also only in a wireless section between the UE 1 and the MeNB 2. In other words, no RRC Connection is present in at least a wireless section between the UE 1 and the SeNB 3. However, the SeNB 3 may create signal information relevant to an RRC message for the UE 1, and may transmit the created signal information to the UE 1 via the MeNB 2.
In addition, examples of a U-plane connection configuration in Dual Connectivity include a Split bearer option configuration and an SCG bearer option configuration.
FIG. 20 illustrates an example of a U-plane connection configuration for the Split bearer option configuration, and FIG. 21 illustrates an example of a Radio Protocol connection configuration for the Split bearer option configuration.
As illustrated in FIGS. 20 and 21, in the case of the Split bearer option configuration, U-plane DL packet data are transmitted from the S-GW 5 to only the MeNB 2, but are not transmitted to the SeNB 3. Note that, in the configurations in FIGS. 20 and 21, a bearer from the MeNB 2 to the UE 1 is referred to as an MCG bearer, and a bearer from the SeNB 3 to the UE 1 is referred to as an SCG bearer.
As illustrated in FIG. 21, each of the UE 1, the MeNB 2, and the SeNB 3 has a layer configuration that is composed of a PDCP (Pacet Data Convergence Protocol) layer, an RLC (Radio Link Control) layer, and a MAC (Medium Access Control) layer.
In the MeNB 2, U-plane DL packet data received from the S-GW 5 are accepted on the PDCP layer. Herein, one PDCP layer (a right-side layer in FIG. 21) of the MeNB 2 can transmit certain part of packet data (PDCP PDU (Protocol Data Unit)) to the UE 1 via a cell under control of its own, and can transmit certain part of packet data (PDCP PDU) to the UE 1 via the SeNB 3. In other words, in the PDCP layer of the MeNB 2, U-plane packet data can be split.
Incidentally, in a wireless communication system implementing Dual Connectivity, an MeNB may change an SeNB (Source SeNB) with which the MeNB is connected, to another SeNB (Target SeNB), along with addition and deletion of an SeNB. This is a technique called SeNB Change. When SeNB Change is performed, a Source SeNB is required to transfer packet data remaining in the Source SeNB to another device.
NPL 1 prescribes that, when SeNB Change is performed in the case of the Split bearer option configuration, packet data remaining in a Source SeNB are transferred to an MeNB. The packet data transferred from the Source SeNB to the MeNB are thereafter transferred from the MeNB to a Target SeNB.